[SI-LIST] : Split planes for DACs?

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From: Greg Erker (erker@sask.trlabs.ca)
Date: Wed Jul 19 2000 - 10:18:36 PDT


Hi all,

  I'm new to the list but have read
a lot of useful info in the archives
of this list. Didn't see an answer
to my question though.

  I'm working on a board that has
four 100 Ms/s 12 bit DACs. I am
powering them from a linear 3.3V
regulator (separate from the digital
3.3V which is coming from a DC-DC
converter).

  So I inherently have a split plane
for the digital 3.3 and DAC 3.3 voltages.
The left 2/3 of the board is digital
circuitry and the right 1/3 has the 4
DACs powered by the linear 3.3V reg.

  I'm also thinking of following the
app note advice and having a split
power and ground planes for the analog
3.3 on each DAC.

  The splits are under the 4 dacs so
that the digital signals don't have
to cross the split plane moats.

  Thus the linear 3.3V will directly
power the digital pins of the DACs.
The four analog 3.3 islands will be connected
to the linear 3.3V with a 1206 component
so I can place an inductor or ferrite
for connection.

  The four ground islands are connected
to the ground plane with a 1206 component
as well (on the bottom side under the DAC).

  Thus far I'm still connecting the ground
plane for the digital 2/3 of the board to
the dac side (except for the 4 small
islands). There are no digital tracks crossing
the linear 3.3/digital 3.3 boundary except
for the dac signals.

  Here are my options:

1) leave things as above.

2) remove a-gnd islands for the 4 dacs.

3) remove a-gnd AND a-3.3 islands for the 4 dacs.

4) change 4 a-gnd and a3.3 islands into two since
pairs of dacs are likely to be running at the same
speed (but not all 4 at same speed).

5) split the ground plane between digital
side and linear regulated side. Probably not
a good idea given the 48 digital signals
crossing the moat.

  The board is going to be at least a 6 layer
with 3 power planes (3.3V, 1.8V and gnd) and
3 signal layers. It may also end up as 8 layers
with 4 sig layers and 4 power layers (3.3/gnd and
1.8/gnd).

Thanks for any help - Greg E.

Staff Researcher
TRLabs Saskatoon

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